The objective of this research is to isolate and characterize deletion mutants of a helper-independent murine sarcoma virus which are unable to complete the replication cycle. Clones of cells carrying the genome of different replication-defective mutants will be isolated and classified by genetic tests. The nature of the virus-specific RNA and the virus speicific proteins present in the clones will then be characterized and compared with those present in cells transformed by the parental virus. The characterization of these mutants will ultimately allow the construction of a deletion map of the RNA tumor virus genome, and the identification of that part of the genome which is involved in transformation.